How much can a graviton be squeezed?

por Nikolaos Mavromatos (IFIC-CSIC University of Valencia)

lunes 27 oct. 2025 11:30 → 12:30 Europe/Madrid

4-4-4426 - Seminari Física Teòrica (Campus Burjassot)

Despite the observation of Gravitational Waves in 2015, a proof that Gravity is quantum in nature still eludes us. An unambiguous proof of Quantum Gravity would be the detection of a single graviton, the quantum field that would serve as a carrier of the gravitational interaction, if the latter exists as a fundamental, and not emergent, force of Nature. However this task, despite excellent ideas, some of which are quite recent, seems to be very difficult to achieve, at least within the framework of the presently available technologies. Nonetheless, while the direct detection of single gravitons remains beyond experimental reach at present, this may not be the case for squeezed quantum graviton states. In a recent article, we have proposed a method for probing quantum gravitational effects via squeezed gravitons produced through astrophysical processes near rotating black holes. The key novelty lies in the exploitation of condensates (“clouds’’) of massive axion-like particle (ALP)  formed via a superradiance instability in the interaction between the ALP and the rotating black hole. The exponential growth of such condensates offers a macroscopic amplification mechanism of potential quantum-gravity effects, due to the very large numbers of ALPs involved in the cloud. This, in turn, enables the generation of sufficiently large numbers of quantum-entangled multi-mode  squeezed graviton states that could be detectable in current or near-future interferometric data. The non-observation by the LIGO–Virgo–KAGRA  interferometers of such squeezed gravitons at present can place (for the first time) stringent upper-bound constraints on ALP cloud lifetimes. The talk will explain how spin-polarised-entangled pairs of squeezed gravitons can be produced through interactions of ALPs, studied within the framework of weak-quantum-graviton effective field theories. It will also be demonstrated explicitly that the structure of the entangled states (when the latter are expressed in a left-right polarization basis, analogous to EPR states in particle physics) depends highly on whether their production occurs via ALP annihilation (due to conventional General-Relativity-type interactions) or decays of ALPs, due to the presence of gravitationally anomalous Chern-Simons interactions of ALPs with gravity, which are non-trivial in the background of a rotating black hole. In the talk I will draw analogies between the above effect and analogous effects of squeezed photons in quantum optics, but I will also stress important differences. This idea combines four frontiers — black hole physics, axion phenomenology, gravitational waves and quantum gravity — in a compelling and testable framework.